The invention relates to a technology for conducting synchronous rectification type DC-to-DC conversion or AC-to-DC conversion.
A synchronous rectification type DC-DC converter has hitherto been employed for a variety of electronic devices. The synchronous rectification type DC-DC converter has a function of reducing DC power obtained through conversion by, e.g., an AC adapter from a commercial power source, DC power obtained from a battery and so on down to a voltage suited to an operation of an internal circuit. This synchronous rectification type DC-DC converter has an advantage that efficiency is high while a loss is small.
A synchronous rectification type DC-DC converter is considered to have, for example, a configuration shown in FIG. 13. The DC-DC converter includes an FET 111 serving as a main switch connected to an input terminal 101, an FET 112 serving as a synchronous rectification switch connected between a source terminal of the FET 111 and the ground, an output coil 113, an output capacitor 114, an input capacitor 115, and an FET driver unit 116 for alternately driving the FET 111 and the FET 112.
In the DC-DC converter in FIG. 13, an oscillation control unit 120 controls the FET driver unit 116 in accordance with outputs of an output voltage detection unit 117, an output current detection unit 118 and an operating state control unit 119, and controls an output voltage by changing a duty ratio of the FET 111.
A conceivable configuration of the DC-DC converter is that in the case of requiring a large load current, for example, as shown in FIG. 14, a plurality of output control FETs 91, 92 are provided and driven by a driver circuit 93, and a plurality of synchronous rectification FETs 94, 95 are provided and driven by a driver circuit 96.
Further, a known configuration is that a multiplicity of driver circuits (circuits of controlling the FETs) is prepared to selectively operate the FETs, thus giving flexibility to fluctuations in load. It is, for instance, a DC-DC converter (ADP3205 (Analog Devices IC) etc.) flexible to multiphase.
This type of flexible-to-multiphase DC-DC converter includes the FET and a coil for every phase. Namely, this DC-DC converter includes n-pieces of output control FETs, n-pieces of synchronous rectification FETs and n-pieces of coils for n-phases.
In this system, if the load current is small, circuit efficiency is improved by stopping the driver for driving the phase on a phase-by-phase basis.
Further, the prior arts related to the invention of the application are the following technologies disclosed in Patent documents 1 and 2.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2003-284333
[Patent document 2] Japanese Patent Application Laid-Open Publication No. 2003-319649.